EP0713112A1 - Optical emitting and receiving device with a surface emitting laser - Google Patents

Abstract

The transmission and reception device has a transmission element, e.g. a vertical cavity surface emitting laser (VCSEL), supported by a first carrier (T1) and a reception element (PD) and a transmission fibre (Fa) supported by a third carrier (T3), with a second carrier (T2) between the first and third carriers. The second carrier is transparent to the light wavelength emitted by the transmission element, with a recess housing a monitoring diode (MD) at the interface between the first carrier and the second carrier.

Description

The invention relates to an optical transmitting and receiving device with a surface-emitting laser according to the preamble of claim 1.

In an optical transmission and reception device, a transmission fiber must be coupled to a transmission element, usually a laser diode, and to a photodiode as the reception element. The transmit and receive signals are simultaneously transmitted in the opposite direction in the transmission fiber. The transmit and receive signals are separated at the same wavelength using a beam splitter and at different wavelengths using a wavelength-selective splitter. In order to obtain the lowest possible coupling losses, the fiber must be optimally coupled to both the laser diode and the receiving diode. In order to couple a laser with a horizontally lying resonator and light exit surface on the front side to a single-mode fiber, a beam transformation must be carried out by both because of the different beam characteristics. An image with one or two lenses is usually used for this. The required magnification ratio M is approximately three to five, depending on the ratio of the mode field diameters of the laser and the fiber. In a newer type of laser diode, the Vertical Cavity Surface Emittig Laser (VCSEL), the cavity and the direction of radiation are perpendicular to the chip surface. The mode field diameter is matched to the mode field diameter of a single-mode fiber in order to achieve good coupling. The enlargement ratio in this case is M = 1. Tolerances in the position of the In a VCSEL, lasers are of the same order of magnitude as the tolerances of a fiber-fiber coupling.

Surface-emitting lasers (VCSEL) are even more sensitive than back-surface-emitting lasers to back reflections on external reflection surfaces in the laser resonator. A very critical reflection surface would be a fiber end face that would be attached directly in front of the light exit window of the laser, as would be necessary to achieve a good coupling efficiency. If a VCSEL is to be used in a transceiver module, an element for directional separation must be inserted between the transmission fiber and the laser. This element cannot be attached to the end face coupling between the laser and the fiber for reasons of space.

From DE 39 14 835 C1 an arrangement for coupling an optical waveguide to an optical transmitting or receiving element is known.

An adjustment in the plane lateral to the optical axis is achieved in that the optical waveguide and the optical transmitting or receiving element are fixed on different carriers, the carrier surfaces of which are displaceable on one another and that the light beam is reflected by two reflections each on a mirror plane on a carrier Optical fiber arrives at the optically active element or vice versa. A lateral adjustment is carried out by moving the carrier. The carrier which carries the transmitting or receiving element can consist of a substrate and a part applied thereon, which has a continuous opening through which the light beam passes. The arrangement can be used in all transmission systems with optical fibers, in duplexers with light coupling in or out. In the case of coupling to a transmission element, in particular to one edge emitting laser, a receiving element can be provided on the carrier with the fiber.

Starting from this prior art, it is an object of the invention to provide an optical transmitting and receiving device in which the adjustment effort is reduced and assembly is simplified.

The object is achieved by an invention with the features of claims 1 and 2.
Advantageous further developments are specified in the subclaims.

A solution is proposed in which all components of a transceiver module can be installed without adjustment using VCSEL and the coupling is carried out without harmful back reflections on the laser. In addition, suggestions are made for a low-reflection coupling of a monitor diode to regulate the light output. The holding structures required here can be produced inexpensively in large-scale micromechanical production.

• Fig. 1 einen Schnitt durch eine erfindungsgemäße Anordnung mit Monitordiode auf dem Träger der Laserdiode;
• Fig. 1a einen Schnitt durch eine erfindungsgemäße Anordnung mit Monitordiode auf dem Träger der Übertragungsfaser;
• Fig. 2 Aufbau zur Justage der Anordnung;
• Fig. 3 erfindungsgemäße Anordnung, wobei der dritte Träger mit seiner Stirnseite zum ersten Träger ausgerichtet ist und
• Fig. 4 erfindungsgemäße Anordnung, deren dritter Träger gegenüber der Anordnung nach Fig. 1 vertikal und horizontal gespiegelt ist.

Embodiments of the invention are described with reference to the drawings. Show it:

• 1 shows a section through an arrangement according to the invention with a monitor diode on the carrier of the laser diode.
• 1a shows a section through an arrangement according to the invention with a monitor diode on the carrier of the transmission fiber;
• Fig. 2 structure for adjusting the arrangement;
• Fig. 3 arrangement according to the invention, wherein the third carrier is aligned with its end face to the first carrier and
• Fig. 4 arrangement according to the invention, the third carrier is mirrored vertically and horizontally compared to the arrangement of FIG. 1.

. Zur Erleichterung der Positionierung wird die Laserdiode bei der Montage an die Fußlinien von mindestens zwei rechtwinklig zueinander liegenden Seitenflächen angelegt.A first embodiment of the solution according to the invention is shown in FIG. 1. In a first carrier T1, which consists of monocrystalline silicon, anisotropic etching produces a depression V1 which has a flat bottom B1, on which a perpendicularly radiating laser diode VCSEL is mounted. Due to the anisotropic etching process, the side surfaces of the depression have an inclination angle of $\text{α = arctan (}\sqrt{\text{2nd}}\text{) = 54.7 °}$

. To facilitate positioning, the laser diode is attached to the base lines of at least two side surfaces that are at right angles to each other.

gebrochen, wobei n_o der Brechungsindex im Außenraum und n₂ der Brechungsindex im Träger T2 ist.Above the carrier T1, a second carrier T2 is attached, which is transparent to the wavelength λ₁ of the laser light. For example, this second carrier can also consist of silicon. However, another transparent material that can be structured micromechanically is also possible. On the underside of the carrier T2, an oblique surface SF is structured in the area above the active surface of the VCSEL, the angle of inclination δ thereof relative to the underside of the carrier T2 is so large that the transmission light bundle L1 emerging vertically from the VCSEL is at the inclined interface SF below the angle γ₁₂ is broken. A lens Li is attached to the top of the carrier T2. This lens can advantageously be a planar Fresnel lens or a holographic lens. However, other types of lenses are also possible, for example a spherical lens which is seated in a micromechanically shaped depression, or a lens produced by dry etching. The lens Li converts the initially divergent light bundle L1 into a convergent bundle. As a result of the refraction of light at the upper boundary surface of the carrier T2, the central beam of the light beam becomes angled $${\text{γ}}_{\text{11}}{\text{= arcsin ((n}}_{\text{2nd}}{\text{/ n}}_{\text{O}}{\text{) * sin (γ}}_{\text{12th}}\text{))}$$

broken, where n _{o is} the refractive index in the exterior and n₂ is the refractive index in the carrier T2.

Hieraus errechnet sich der Winkel γ₁₂ zu 5,5°, wenn für den Träger T2 der Brechungsindex von Silizium mit 3,4777 (λ = 1550 nm) eingesetzt wird. Der Neigungswinkel δ, der erforderlich ist, um an der Grenzfläche SF den Winkel γ₁₁ = 5,5° zu erzeugen, errechnet sich aus der transzendenten Gleichung $${\text{(n}}_{\text{2}}{\text{/n}}_{\text{o}}{\text{)*sin(δ - γ}}_{\text{11}}\text{) - sin δ = 0}$$

Durch Iteration erhält man mit den oben angegebenen Werten für n₂, n_o und γ₁₁ einen Neigungswinkel von δ = 8,71°.A further carrier T3 is attached above the carrier T2 and, like the carrier T1, likewise consists of single-crystal silicon. Two wells V31 and V32 are anisotropically etched in this carrier T3. The depression V31 is a V-groove for receiving the transmission fiber Fa. The width of this V-groove is expediently so large that the bottom surface line of the fiber comes to lie just in the plane of the underside of T3. The end face S3 of the V-groove is coated with a wavelength-selective filter Fi1. This filter is designed so that the transmission wavelength λ₁ reflects and the reception wavelength λ₂ is transmitted. The transmitted light bundle L1 is then reflected on the end face S31 inclined at the angle α and coupled into the transmission fiber Fa if the angle γ₁₁ obeys the following relationship: $${\text{γ}}_{\text{11}}\text{= 2 * α - 90 ° = 19.5 °}$$

From this, the angle γ₁₂ is 5.5 ° if the refractive index of silicon with 3.4777 (λ = 1550 nm) is used for the carrier T2. The angle of inclination δ, which is required to generate the angle γ₁₁ = 5.5 ° at the interface SF, is calculated from the transcendent equation $${\text{(n}}_{\text{2nd}}{\text{/ n}}_{\text{O}}{\text{) * sin (δ - γ}}_{\text{11}}\text{) - sin δ = 0}$$

By iteration, the values for n₂, n _o and γ₁₁ given _above give an angle of inclination of δ = 8.71 °.

Instead of refraction on the inclined surface, the angle γ₁₂ required for the correct light guidance can also be achieved by other means. For example, a refractive or diffractive can be on the surface of the VCSEL Deflection element such as a Fresnel element or a hologram are applied, which deflects the direction of light by the angle γ₁₁, so that the angle γ₁₂ is achieved by refraction on the underside of the carrier T2 in the carrier T2. Another possibility is to mount the VCSEL chip inclined at the angle γ₁₁. This can be done by tilting the bottom of the recess V1.

Since the mode field diameters of VCSEL and fiber are of approximately the same size, the lens Li must be designed in such a way that a 1: 1 image with M = 1 is produced. Any differences in the mode field diameters can be easily compensated for by adapting the imaging ratio. For a magnification ratio of M = 1, the optical path lengths for the object and image width must also be the same. When calculating the optical path lengths, the refractive index of the material passed through must be taken into account. The object width can be adapted to the image width in the space in front of the fiber Fa by the choice of the thickness of the carrier 2. Since the coupling tolerances of a VCSEL are the same as the fiber coupling tolerances, there is no need for active adjustment. The coupling tolerances in the lateral direction for fiber and VCSEL are approx. 2 - 3 µm. In the axial direction they are around 30 µm. These tolerances are to be maintained by means of micromechanically structured silicon holding structures, so that adjustment-free assembly is possible. For the adjustment-free assembly of the three carriers T1-T3 on one another, micromechanically generated stops or depressions Va are used, into which adjustment bodies JK are placed.

wobei β₃ der Brechungswinkel an der Stirnfläche S31 mit $${\text{β}}_{\text{3}}{\text{= arcsin((n}}_{\text{o}}{\text{/n}}_{\text{3}}\text{)*sin(90°-α))}$$

ist, gegen die Flächennormale der Substratoberfläche von T3 in das Silizium hineingebrochen. Dabei ist n_o der Brechungsindex in der V-Nut V31 und n₃ = 3,4777 der Brechungsindex im Siliziumträger T3. Mit n_o = 1 für Luft erhält man β₃ = 9,6° und γ₂₁ = 64,3°. Das Lichtbündel L2 trifft auf die Seitenfläche S31 der Vertiefung V31 unter einem Einfallswinkel von $${\text{α}}_{\text{3}}{\text{= 180° -2*α - β}}_{\text{3}}\text{= 61,0°.}$$

The receiving light bundle L2 emerging from the transmission fiber with the wavelength λ₂ penetrates the filter Fi1 and becomes at the boundary to the silicon at an angle $${\text{γ}}_{}$$$${}_{\text{21}}{\text{= α + β}}_{\text{3rd}}\text{,}$$

where β₃ the angle of refraction on the end face S31 with $${\text{β}}_{\text{3rd}}{\text{= arcsin ((n}}_{\text{O}}{\text{/ n}}_{\text{3rd}}\text{) * sin (90 ° -α))}$$

is broken into the silicon against the surface normal of the substrate surface of T3. Here, n _{o is} the refractive index in the V-groove V31 and n₃ = 3.4777 the refractive index in the silicon carrier T3. With n _o = 1 for air one obtains β₃ = 9.6 ° and γ ₂₁ = 64.3 °. The light bundle L2 strikes the side surface S31 of the depression V31 at an angle of incidence of $${\text{α}}_{\text{3rd}}{\text{= 180 ° -2 * α - β}}_{\text{3rd}}\text{= 61.0 °.}$$

ist, wird das Lichtbündel L2 unter dem Winkel $${\text{γ}}_{\text{22}}{\text{= α}}_{\text{3}}\text{- α = 6,3°}$$

gegen die Flächennormale der Trägeroberfläche gebrochen. Der Winkel γ₂₂ ist kleiner als α_g, so daß das Lichtbündel L2 auf der Oberfläche des Siliziumträgers T3 austreten kann. An der Austrittsstelle des Lichtbündels L2 wird die Empfangsdiode PD montiert. Die Position für die Photodiode ergibt sich aus den oben genannten Winkeln, dem Abstand der beiden Vertiefungen V31 und V32 voneinander und mit geringer Abhängigkeit von der Dicke des Trägers T3. Die Position der Lichtaustrittsfläche von L2 hängt dagegen nicht von der axialen Position der Faser Fa in der V-Nut V31 ab. Die Position der Lichtaustrittsfläche kann daher relativ zu den mikromechanisch erzeugten Vertiefungen V31 und V32 durch Marken oder Anschläge gekennzeichnet werden. Diese Marken oder Anschläge können durch photolithographische Technik sehr genau zu den Vertiefungen V31 und V32 ausgerichtet werden.Since this angle α₃ is greater than the critical angle of total reflection at the silicon / air transition from $${\text{α}}_{\text{G}}{\text{= arcsin (n}}_{\text{O}}{\text{/ n}}_{\text{3rd}}\text{) = 16.7 °}$$

is the light beam L2 at the angle $${\text{γ}}_{\text{22}}{\text{= α}}_{\text{3rd}}\text{- α = 6.3 °}$$

broken against the surface normal of the carrier surface. The angle γ₂₂ is smaller than α _g , so that the light bundle L2 can emerge on the surface of the silicon carrier T3. The receiving diode PD is mounted at the exit point of the light bundle L2. The position for the photodiode results from the above-mentioned angles, the distance between the two depressions V31 and V32 from one another and with little dependence on the thickness of the carrier T3. The position of the light exit surface of L2, however, does not depend on the axial position of the fiber Fa in the V-groove V31. The position of the light exit surface can therefore be relative to the micromechanically generated recesses V31 and V32 Marks or characters are marked. These marks or stops can be aligned very precisely with the depressions V31 and V32 using photolithographic technology.

To control the light output of the transmitter laser, the laser line must be measured with a monitor diode MD. The monitor diode must also be coupled with little reflection. According to the invention, a narrow V-groove Vm is anisotropically etched under the laser diode VCSEL. One end face Sm1 of this V-groove lies below the lower light exit surface of the VCSEL and the other end surface Sm2 lies below the monitor diode MD attached next to the VCSEL. The side walls of the monitor V-groove Vm are mirrored, so that the monitor signal reaches the monitor diode after several reflections.

gegen die Flächennormale auf die Oberfläche des Trägers T3 auf. Dieser Winkel ist aber größer als der Grenzwinkel der Totalreflexion α_g = 16,7°, so daß das direkte Störlicht vom Sender nicht in die Empfangsdiode gelangen kann.Another advantage of the solution according to the invention is that a very high near crosstalk attenuation can be achieved. A high near crosstalk attenuation is necessary so that the transmitted signal from the laser does not hit the receiving diode located near the transmitter due to insufficient directional separation and disturbs the reception of weak useful signals. Filter layers generally have a limited ability to separate different wavelengths. Therefore, a small proportion of the transmitted light bundle L1 will also penetrate the filter layer S31. The beam path of this stray light is shown in dashed lines as S1 '. This light beam hits at an angle $${\text{γ}}_{\text{13}}{\text{'= α - β}}_{\text{3rd}}\text{= 45.2 °}$$

against the surface normal on the surface of the carrier T3. However, this angle is larger than the critical angle of the total reflection α _g = 16.7 °, so that the direct interference light from the transmitter cannot reach the receiving diode.

In a variant of the first exemplary embodiment, the filter Fi1 is designed such that a small part of the transmitted light still penetrates the filter while the largest part is reflected. This light beam L1 'penetrating the filter is used according to the invention as a control signal. The monitor diode MD 'is then not mounted on the carrier T1 but in a recess V33 on the carrier T2. This is shown in dashed lines in FIG. 1.

An active adjustment of the fiber to the transmitted light bundle L1 can be dispensed with because of the larger mode field of a VCSEL compared to a face-emitting laser. The required accuracy in the range from 2 to 3 μm is achieved by adjustment bodies JK in depressions Va or by micromechanically generated stops in the surfaces of the supports T1 to T3 lying opposite one another. The carrier T2 can serve as a translucent, hermetically sealed cover of the housing G. An additional hermetically sealed window Fe can also be used between the supports T2 and T3 (see FIG. 2).

In a second exemplary embodiment of the solution according to the invention, the carrier T3 is not aligned with its underside but with its end face to the carrier T2. The second exemplary embodiment according to the invention is shown in FIG. 3. The carriers T1 and T2 are constructed as in the first exemplary embodiment. The fiber Fa is again guided in a V-groove V31 in a carrier T3 and is also axially adjustable in this V-groove. The end face S31 is also covered with a wavelength-selective filter layer Fi2. In contrast to the filter layer F11 in the first exemplary embodiment, the filter layer Fi2 is transparent for the transmission wavelength λ₁ and reflective for the reception wavelength λ₂. The emerging at an angle of γ₁₁ = 19.5 ° from the beam T2 light beam L1 strikes the side wall S31 one of the opposite side in the carrier T3 anisotropically etched recess V32, the part opposite the side wall S32 has been removed, for example by sawing. Since the two side surfaces S32 and S31 are parallel to one another, the transmitted light bundle S1 is offset in parallel by the double refraction and then hits the transmission fiber Fa. In contrast to the first exemplary embodiment, the light beam direction in the carrier T2 need not be inclined here by special measures, but can stay vertical. The carrier T3 is then mounted perpendicular to the surface normal of the carrier T2. The receiving diode PD is mounted over the end face S31 of the fiber V-groove V31. In this exemplary embodiment, the received signal emerging from the fiber has only a very short light path until it hits the receive diode. As a result, it fans out less so that a smaller-area photodiode can be used.

A third embodiment of the solution according to the invention is shown in FIG. 4. Here, the carrier T3 is constructed similarly to the first exemplary embodiment, but is mirrored vertically and horizontally compared to the first exemplary embodiment. As in exemplary embodiment 2, the filter layer Fi2 must be transparent for the transmission wavelength and reflective for the reception wavelength. As in exemplary embodiment 2, the photodiode PD for the received signal is mounted in the region above the end face of the V-groove V31. As in embodiment 2, the advantage here is that the path between the end face of the fiber and the photodiode is very short, which results in a small beam expansion and therefore allows a very small-area photodiode that is suitable for high frequencies. The directional angle γ₁₂ of the beam in the carrier T2 is 5.5 ° for the carrier material silicon. At this directional angle is adjusted by measures, as described in the first embodiment, the directional angle γ₂₁ of the beam in the carrier T3. The one opposite the first Embodiment here longer light path in the carrier T3 for the transmitted light beam must be compensated for by a corresponding thickness of the carrier T2.

The carrier T2 with the lens Li is produced in large-scale use for many individual modules and all lenses are assembled together with the lasers in a single adjustment and assembly process. Passive adjustment using marks or adjustment-free assembly using micromechanically structured stops is possible here. The depressions V2 in the carrier T2 are designed such that the optoelectronic and electronic components such as the laser diode LD, the monitor diode MD or electronic modules (not shown here) for controlling the laser are hermetically sealed. After the carrier substrates T1 and T2 have been joined together, they are separated by sawing or by breaking on micromechanically generated predetermined breaking lines. The position of the sawing or breaking lines is such that the position of the depressions V1 and V2 and the lenses Li are not touched.

The monitor diode MD can also be mounted on the underside or top of the supports T2 or T3, with corresponding recesses being provided in the adjacent support. A further lens Lim can be provided on the carrier T2 for coupling the monitor diode.

Claims (6)

Optical transmission and reception device with a transmission element (LD), which is fixed on a first carrier (T1), with a receiving element (PD) and a transmission fiber (Fa), which are fixed on a third carrier (T3) and with a second Carrier (T2), which is located between the first and the third carrier (T1, T3), with V-grooves and depressions in the carriers (T1, T2, T3), which are produced by anisotropic etching, with at least one mirror surface on third carrier (T3), characterized in
that the third carrier (T3) is transparent to light with the wavelength of the light emitted by the transmitting element (LD), that a monitor diode (MD) is provided which is on the surface of the first carrier (T1) in a recess of the second carrier (T2 ) is mounted,
that the transmitting element (LD) is a surface emitting laser diode and
that the arrangement between the refractive surface of the second carrier (T2) and the transmitting element is selected such that the beam hits the mirror surface (S31) through the second carrier (T2) and from there onto the transmission fiber (Fa). Optical transmission and reception device with a transmission element (LD), which is fixed on a first carrier (T1), with a receiving element (PD) and a transmission fiber (Fa), which are fixed on a third carrier (T3) and with a second Carrier (T2), which is located between the first and the third carrier (T1, T3), with V-grooves and depressions in the carriers (T1, T2, T3), which are produced by anisotropic etching, with at least one mirror surface on third carrier (T3), characterized in that the third carrier (T3) for light with the wavelength of the light emitted by the transmitting element (LD), it is transparent that a monitor diode (MD) is provided which is mounted on the surface of the second carrier (T2) in a recess in the third carrier (T3), that the transmitting element (LD ) is a surface emitting laser diode and that the arrangement between the refractive surface of the second carrier (T2) and the transmission element is selected such that the beam through the second carrier (T2) onto the mirror surface (S31) and from there onto the transmission fiber ( Fa) meets. Optical transmitting and receiving device according to claim 1, characterized in that the optical axis of the transmission fiber encloses an angle of 90 ° with the surface of the carrier (T1, T2). Optical transmitting and receiving device according to one of Claims 1 to 3, characterized in that a beveling of the surface of the second carrier (T2) is provided on the second carrier (T2) in the region of the entry of the beam from the transmitting element (LD). Optical transmission and reception device according to one of Claims 1 to 3, characterized in that a hologram or a Fresnel lens is provided on the emitting surface of the transmission element. Optical transmitting and receiving device according to one of Claims 1 to 3, characterized in that the transmitting element (LD) is attached to the carrier (T1) at an angle to the surface of the carrier.

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